1. Field of the Invention
This invention is directed to an apparatus, system and method for growing crystals of a substance from its liquid phase. The crystals grown in accordance with this invention have highly-ordered atomic structures with few impurities or defects, and thus are suitable, for example, for the production of wafer substrates used for the manufacture of semiconductor or optical devices or the like, or for the production of superconductive materials.
2. Description of the Related Art
For decades, researchers have attempted to grow high-purity crystals of substances that undergo peritectic reactions, from a melt. These attempts have proved unsuccessful, however, because substances that behave peritecticly, by their nature, decompose upon solidification from the liquid phase into non-stoichiometric compositions from which a crystal cannot be grown without significant lattice defects due to local variations of the quantities of the various elements and/or compounds that compose the substance. The difficulties of growing crystals of peritectic substances from a melt have been widely recognized. For example, one researcher has noted that the " . . . freezing of a peritectic reaction (or compound), to complete equilibrium, is practically impossible to attain." W. J. Boettinger, Mettalurgical Transactions Vol. 5, pg. 2023, published 1974. Another commentator noted that researchers are exhibiting "a very lively interest" in peritectic reactions, but the ability to control these reactions has alluded many. D. H. St. John, L. M. Hogan, Acta Metallurgica Vol. 25, pg. 77, published 1977. Even more recently, a textbook indicates that many substances decompose peritecticly when melted into a non-stoichiometric liquid with an indeterminate ratio of components or elements. James F. Shackleford, Introduction to Materials Science for Engineers, Macmillan Publishing Company, New York, Third Edition, pg. 222, published 1992. Similar observations regarding the freezing of substances that undergo peritectic reactions have been made in a number of publications. See, for example, J. P. Schaffer, A. Saxena, S. D. Antolovich, T. H. Sanders, Jr., S. B. Warner, The Science and Design of Engineering Materials, Richard D. Irwin, 1995, pp. 274-76; R. Glardon, W. Kurz, Journal of Crystal Growth 51, 283, published 1981; M. Hillert, Solidification and Casting of Metals, The Metal Society, London, published 1979; D. H. St. John, L. M. Hogan, Acta Metallurgica 35, 171, published 1987; F. N. Rhines, Phase Diagrams in Metallurgy, McGraw Hill, New York, 1956, pp. 85-88.
FIG. 1 is a phase diagram explaining the behavior of a substance which undergoes a peritectic reaction as the substance is cooled at constant pressure and volume from the liquid- or-type gas- to the solid-phase. As used in this document, a peritectic (or synonymously, a `peritectic- type` reaction) is defined as a reaction " . . . in which one phase decomposes with rising temperature into two new phases." Frederick N. Rhines, Phase Diagrams in Metallurgy: Their Development and Application, McGraw-Hill Book Company, New York, Pg. 83, published 1956. Thus, although some scientists in this field refer to a `peritectic reaction` as one in which a solid decomposes with rising temperature into a solid and a liquid, the term `peritectic` as used in this document consistent with Rhines'definition, refers to decomposition with rising temperature from any one phase into any two phases. The substance of FIG. 1 includes components A and B that can be elements or compounds, contained within a specific volume at a specific pressure. With substances that contain more than two components, many different phases are possible, so the substance in FIG. 1 is a relatively simple example provided for ease in illustrating a substance that undergoes a peritectic reaction.
At the start temperature, the peritectic substance is in the liquid or gas phase with a composition AB, where A and B denote respective proportions of the element(s) or compound(s) that comprise the peritectic substance. As the substance cools to the peritectic temperature T.sub.p, the element or compound B begins to precipitate as a solid out of the liquid or gas phase, causing the remaining liquid or gas to become relatively rich in A and depleted in B. As more heat is extracted from the liquid substance, the B component of the peritectic substance continues to precipitate or sublimate out of the liquid or gas phase until the proportion of A to B reaches a composition A'B'determined by the shape of the phase boundary. At this point, as further heat is extracted from the substance, the remaining liquid or gas will precipitate or sublimate to form a solid composition A'B'that is richer in A and depleted in B relative to the initial composition AB.
From FIG. 1, the problem presented when attempting to solidify a substance which undergoes a peritectic reaction can be readily understood. Assuming that the initial composition AB was selected with a stoichiometric proportion of A to B, the behavior of the composition as heat is extracted from the system results in the initial formation of component B and the eventual formation of the non-stoichiometric composition A'B'. The resulting solid phase will therefore contain inclusions of B in a matrix of composition A'B', which is extremely unlikely to have the same physical and/or chemical properties as the desired compound AB.
There are many crystalline substances which undergo a peritectic reaction at atmospheric pressure, that are significantly important (or have significant promise of being important) for use in a wide variety of industries. For example, substances such as aluminum nitride (AlN), silicon carbide (SiC), gallium nitride (GaN), yttrium barium cupric oxide (YBa.sub.2 Cu.sub.3 O.sub.x) and aluminum gallium nitride (AlGaN.sub.x) are known or believed to have significant uses for a wide variety of semiconductor, optical or superconductor applications, if available in highly pure crystalline form. With respect to gallium nitride (GaN), one writer has commented that "gallium nitride substrates are the great hope of the nitride community . . . ", and that " . . . good results obtained with nitride light emmiting diodes! would be immediately pushed forward in one large step if high quality gallium nitride substrates were to become available." G.W. Wicks, Growing Interest in Nitrides, Compound Semiconductor, v.2, n.1, pg. 42, January-February 1996. However, due to the extremely slow production rates (typically on the order of microns per hour) at which these peritecticly-behaving substances can be produced using vapor growth techniques, these substances are relatively scarce and expensive. Therefore, the benefits derived through the use of these high-purity, crystalline substances, have been greatly limited by the general unavailability of these substances at reasonable cost. Accordingly, there is a strong need for a device or technique for rapidly growing high-purity, crystalline substances that undergo peritectic reactions if they are attempted to be grown with conventional devices or techniques.
Another problem related to this invention involves safety concerns posed by many crystal-growing devices. More specifically, the furnace chamber surfaces of a crystal-growing device are heated to extremely high temperatures, and pressurized to relatively high pressures, to grow many kinds of substances in vapor phase processes. If the external surfaces of the furnace chamber are not cooled, these heated surfaces can become weakened and rupture under the pressure of the gas contained in the furnace chamber, thus presenting a significant risk of injury to persons in a work environment by explosion of the furnace. To reduce the danger of injury, cooling equipment has been used to cool the furnace chamber surfaces of a crystal-growing device. However, the cooling equipment system generally adds significant expense and complexity to the crystal-growing device. It would therefore be desirable to provide a crystal-growing apparatus that operates with relatively cool external surfaces, but which does not require additional cooling equipment and the expense and complexity associated therewith.
Still another problem related to this invention pertains to energy consumption required in previously known crystal-growing devices. Specifically, in order to produce the extremely high temperatures needed to melt a charge material used to grow a crystal, the typical crystal-growing device consumes considerable power. The power consumption problem of the typical crystal-growing device is exacerbated by the fact that these devices often must be operated continuously for several hours or even days to produce a single crystal. Therefore, the power consumed in previously known crystal-growing devices has added significantly to the cost of producing crystals with typical crystal-growing devices. It would be extremely desirable to reduce the amount of power required to produce a crystal.